SOILS, PLANT GROWTH AND CROP PRODUCTION – Vol.III - Perennial Energy Crops: Growth and Management - M. Weih
PERENNIAL ENERGY CROPS: GROWTH AND MANAGEMENT
M. Weih
Swedish University of Agricultural Sciences, Uppsala, Sweden
Keywords: Agriculture, biomass production, biodiversity, breeding, energy crops,
environment, Miscanthus, Salix, willow.
Contents
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1. Introduction
2. Types of Energy Crops
2.1. Annual and Perennial Energy Crops
2.2. Perennial Crops for Warm and Cool Climates
3. Species
3.1. Miscanthus
3.2. Salix
4. Production Systems and Management
4.1. Miscanthus
4.2. Salix
5. Breeding
5.1. Miscanthus
5.2. Salix
6. Environmental Aspects
6.1. Soil and Water
6.2. Biodiversity
6.3. Landscape
7. Conclusions
Glossary
Bibliography
Biographical sketch
Summary
The production of biomass for energy purposes on agricultural land is an important way
to generate renewable energy, and energy crops will probably be grown on a significant
and increasing fraction of agricultural land in Europe and worldwide. Among the energy
crops, particularly perennial grasses and trees could contribute substantially to
alleviation of global problems in climate change and energy security, if high yields can
be achieved.
Miscanthus and Salix are frequently regarded as front-runners in the search for
commercially viable perennial energy crops as they are high yielding, can cope with a
range of soils and climate conditions, and have a productive life of 15 to 20 years. This
chapter briefly describes growth, management and breeding aspects of perennial energy
crops, in particular Miscanthus and Salix. In addition, environmental aspects including
influence of perennial crops on soil and biodiversity are discussed.
©Encyclopedia of Life Support Systems (EOLSS)
SOILS, PLANT GROWTH AND CROP PRODUCTION – Vol.III - Perennial Energy Crops: Growth and Management - M. Weih
1. Introduction
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Energy consumption worldwide has increased much faster than the increase in
population size. Global problems due to climate change and energy security have
greatly increased the interest for renewable energy including energy crops grown on
agricultural land. In addition, economic evaluations have revealed that energy crop
cultivation has the potential to offer farmers a profitable alternative to frequently
declining returns from conventional land use. Thus, a significant and increasing fraction
of agricultural land in Europe and worldwide is likely to be dedicated to the culture of
energy crops in the near future. There is a great challenge and concern, that cultivation
of energy crops might reduce land availability for feed and food production. However,
experience shows that energy crop production for existing markets does not necessarily
come at the expense of food crops and that it even may contribute to improve food
security (FAO 2008).
2. Types of Energy Crops
2.1. Annual and Perennial Energy Crops
Today, annual crops, i.e. crops grown for one growing season such as maize and wheat,
contribute greatly to bio-energy worldwide, but yields of these crops are highly
dependent on high N inputs into soils. In contrast, perennial grasses and tree crops, i.e.
crops with a life span of more than 2 yr, can achieve higher biomass yields with
relatively lower inputs of nitrogen fertilizer and are regarded as most efficient nitrogen
users. This difference greatly affects energy balance, i.e. energy input per harvest, and
life-cycle analyses of bio-energy chains for the different crops, because the production
of nitrogen fertilizers is very energy intensive.
Figure 1. Perennial biomass crops – from feedstock to end use.
©Encyclopedia of Life Support Systems (EOLSS)
SOILS, PLANT GROWTH AND CROP PRODUCTION – Vol.III - Perennial Energy Crops: Growth and Management - M. Weih
Energy crops are generally grown for two different markets, i.e., solid bio-fuels for heat
and power generation on one hand, and liquid transport fuels on the other hand (Figure
1). Regarding the transport sector, so-called first-generation bio-fuels such as bio-diesel
and bio-ethanol (mainly produced from annual crops such as oilseed rape and cereals)
dominate the bio-fuel sector today. A shift towards second-generation bio-fuels,
produced from mainly ligno-cellulosic biomass (mostly perennial crops such as
Miscanthus and Salix grown in short rotation), is expected for the future. This shift will
greatly affect not only agriculture, but also industry. For example, the development of
perennial biomass crops has been stated to be the major driver for the cellulosic ethanol
industry.
2.2. Perennial Crops for Warm and Cool Climates
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The greatest potential for high biomass yields of perennial energy crops is in areas with
warm and humid climatic conditions supporting rapid plant growth, e.g., many warmtemperate, sub-tropical and tropical climates. Most appropriate crop species to be grown
under these climate conditions are perennial grasses such as Miscanthus and sugarcane,
but also fast-growing trees such as eucalypts and poplars.
Also cool-temperate and boreal floras include fast-growing perennial species that are
suitable for energy plantations in high-latitude regions, where particularly the frequent
occurrence of frost during winter sets limits for the culture of many otherwise highyielding crops. Particularly tree species of Populus and Salix, but also grasses such as
reed canary grass are suitable crops under these conditions, and they are used as energy
crops in northern Europe and North America.
Miscanthus and Salix are often regarded as front-runners in the search for commercially
viable perennial biomass crops, as they are relatively high yielding, can cope with a
range of soils and climate conditions, have relatively few pests and have a productive
life of around 15 to 20 yr.
Miscanthus and Salix represent two different growth habits and life forms, i.e., grasses
and trees, which has major implications for the choice of production system and
management. For example, harvest of grasses is often performed with mowers that are
common in conventional agriculture, whereas tree harvest often requires specialized
machinery (see Section 4.2 below).
Among the energy crops currently discussed for large-scale biomass production on
agricultural land in many regions of the world, Miscanthus and Salix are selected here to
represent the two major groups of perennial energy crops, i.e. grasses and trees.
This chapter briefly characterizes mainly these two species – they may stand for many
other crops of similar life form and appearance – with respect to their potential use as
energy crops on agricultural land and gives an outline of the appropriate production
systems and management actions required to achieve high biomass yields. In addition,
breeding issues of these perennial crops and critical environmental concerns associated
with their culture are addressed.
©Encyclopedia of Life Support Systems (EOLSS)
SOILS, PLANT GROWTH AND CROP PRODUCTION – Vol.III - Perennial Energy Crops: Growth and Management - M. Weih
3. Species
There are a number of perennial plants (grasses and trees) with a great potential as
energy crops and with regionally greater potential compared to Miscanthus and Salix.
The most prominent example is probably sugarcane (species of Saccharum), which is
the basis for the production of liquid transport fuels (ethanol) in Brazil. Several other C4
perennial rhizomatous grasses, e.g. switchgrass (Panicum virgatum L.), species of
Cyperus and Spartina and - not least - fast-growing bamboos (various species within
the family Poaceae) are interesting alternatives for biomass production in warmtemperate and tropical regions.
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Reed canary-grass (Phalaris arundinacea) is an example for a robust perennial C3
grass, widely grown across temperate regions of Europe, Asia and North America,
where it is currently tried as energy crop on agricultural land in some regions, like in
Sweden and Finland.
Among the trees, some species can be grown in short rotation on agricultural land and
are therefore potentially interesting energy crops for agriculture. Apart from Salix,
poplars and aspens (species of Populus) have a great potential as energy crops in
temperate and boreal agriculture, whereas fast-growing eucalypts (species of
Eucalyptus) are sensitive to frost and therefore confined to warm-temperate and tropical
agriculture.
Only Miscanthus and Salix are presented here more in detail, but production systems
and general management issues for many other energy crops are not principally
different from these two species. For example, poplars and aspens are currently much
more important for biomass production than willows in many regions of Europe and
North America, but production systems and management of poplars for energy purpose
is very similar to willow, especially when grown in short rotation.
3.1. Miscanthus
Miscanthus species are tropical and subtropical grasses native to southeastern Asia,
China, Japan, Polynesia and Africa. Cultivars of M. sacchariflorus, M. sinensis, their
hybrids, and other Miscanthus species are frequently grown as ornamental crops. The
genotype with the greatest biomass potential is a sterile hybrid (Miscanthus x
giganteus), likely of M. sacchariflorus and M. sinensis parentage.
Miscanthus species are similar in appearance to some types of bamboo, although they
are more closely related to sugarcane. They produce a new canopy of tall canes each
year. In their region of origin, the canes can reach a height of 7 to 10 m, whilst in
Europe heights of around 4 m are more common (Fig. 2). They have a rigorous root and
rhizome system that can penetrate to depths of 1 m or more. The very high potential
yield of Miscanthus is due partially to its C4 photosynthetic pathway, which is similar
to other plants of tropical origin such as maize, sugarcane and sorghum. Compared to
the C3 photosynthetic pathway, which is common for most agricultural crops native to
the temperate zone (e.g. wheat), the C4 photosynthetic pathway typically enables the
©Encyclopedia of Life Support Systems (EOLSS)
SOILS, PLANT GROWTH AND CROP PRODUCTION – Vol.III - Perennial Energy Crops: Growth and Management - M. Weih
plants to employ a more efficient carbon fixation process, particularly when grown
under warm and droughty conditions.
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Although having evolved in regions experiencing warm temperatures and high
precipitation, Miscanthus has been grown successfully in temperate regions and has
demonstrated an above ground tolerance for light frost conditions. Leaves and shoots
are destroyed below approximately -5oC, but the plants can overwinter as rhizomes at
considerably lower temperatures. In comparison to most crops native to temperate
climates, the temperature requirements to break winter dormancy of the tested
Miscanthus genotypes are very high, which strongly limits productivity in temperate
regions by means of greatly delayed spring emergence and growth. Delayed spring
growth in combination to frost sensitivity are likely to be the major limiting factors for
Miscanthus culture in environments with short growing seasons and cold winters.
Figure 2. Miscanthus giganteus is a rhizomatous grass that can be grown in very dense
stands, here at Woburn Experimental farm, Rothamstead Research, UK
(Photo: A. Riche, Rothamstead Research)
In Europe and North America, Miscanthus cropping was tried since the late 1990s in
field experiments to investigate agronomic problems such as plant establishment,
nutrient supply, yield development and biomass properties. Mean harvestable biomass
yields over several harvest years are frequently reported at around 10 to 15 tons ha-1 yr1
, although figures beyond 20 tons ha-1 yr-1 were also reported. Compared to many
annual crops, Miscanthus cropping has also been reported to enhance the nutrient
cycling in the plant–soil system. For example, as a result of high inputs of leaves,
rhizomes and roots, increased concentration of organic carbon and total nitrogen have
been demonstrated.
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Bibliography
Clifton-Brown, J.C., Stampfl, P.F. and Jones, M.B. (2004). Miscanthus Biomass Production for Energy in
Europe and its Potential Contribution to Decreasing Fossil Fuel Carbon Emissions. Global Change
Biology, 10: 509-518. [This document describes the potential contribution of Miscanthus culture to
ameliorate the consequences of climate change.]
DEFRA [Dept. for Environment, Food and Rural Affairs] (2008). Planting and Growing Miscanthus Best Practice Guidelines. http://www.defra.gov.uk/erdp/pdfs/ecs/miscanthus-guide.pdf. [This booklet is
designed to introduce farmers to the energy crop Miscanthus]
Gibson, L. and Barnhart, S. (2007). Miscanthus Hybrids for Biomass Production. Iowa State University,
Dept. of Agronomy (www.agron.iastate.edu), 2 pp. [This document very briefly describes Miscanthus
agriculture from a North American perspective.]
FAO (2008). The State of Food and Agriculture 2008. Bio-fuels: Prospects, Risks and Opportunities.
FAO, Rome, Italy. 138 p. [This document surveys the current state of the debate on bio-fuels and explores
its implications for the environment, food security and the poor.]
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Larsson, S., Nordh, N.-E., Farrell, J. and Tweddle, P. (2007). Manual for SRC Willow Growers.
Lantmännen Agroenergi AB (www.agroenergi.se), Örebro, Sweden. 18 p. [This document is a grower’s
manual for Salix culture on agricultural land.]
Hein, K.R.G. (2005). Future Energy Supply in Europe – Challenges and Chances. Fuel, 84: 1189-1194.
[This document reviews the current discussion on energy supply and the need for energy crops from a
European perspective.]
Kahle, P., Beuch, S., Boelcke, B., Leinweber, P. and Schulten, H.-R. (2001). Cropping of Miscanthus in
Central Europe: Biomass Production and Influence on Nutrients and Soil Organic Matter. European
Journal of Agronomy, 15: 171-184. [This document describes Miscanthus culture and its effects on soils.]
Karp, A. and Shield, I. (2008). Bioenergy from Plants and the Sustainable Yield Challenge. New
Phytologist, 179: 15-32. [This document gives an up-to-date overview on the biology of energy crops
grown on agricultural land.]
Styles, D., Thorne, F. and Jones, M.B. (2008). Energy Crops in Ireland: An Economic Comparison of
Willow and Miscanthus Production with Conventional Farming Systems. Biomass and Bio-energy, 32:
407-421. [This document is a case study of the economic result of Salix and Miscanthus culture in
comparison to conventional agricultural crops.]
Weih, M. (2004). Intensive Short Rotation Forestry in Boreal Climates: Present and Future Perspectives.
Canadian Journal of Forest Research, 34: 1369-1378. [This document describes the specific problems and
possibilities associated with Salix short rotation forestry in cool-temperate regions.]
Weih, M. and Nordh, N.-E. (2007). Biomass Production with Fast-growing Trees on Agricultural Land in
Cool-temperate Regions: Possibilities, Limitations, Challenges. In: Environmental Research Advances.
New York, Nova Science Publishers, pp. 155-170. [This document describes the possibilities and
limitations of Salix agriculture from both commercial and environmental perspectives.]
Wright, L. (2006). Worldwide Commercial Development of Bio-energy with a Focus on Energy Cropbased Projects. Biomass and Bio-energy, 30: 706-714. [This document describes the development of
energy crops worldwide during the late 1990s and early 2000s.]
Biographical Sketch
Martin Weih, PhD, is a Professor of crop eco-physiology at the Department of Crop Production Ecology
at the Swedish University of Agricultural Sciences (SLU) in Uppsala. He has more than 15 years of
experience in ecological research on birch, willow, poplar, bamboo and other species. His main interest is
in plant eco-physiology and ecological questions related to biodiversity. Major research activities focus
on the production ecology and physiology of energy crops - mainly perennial biomass crops such as Salix
and Populus grown on agricultural land - and the genetic basis of production traits. Effects of plantations
of fast-growing trees on the environment (e.g., biodiversity) and landscape are another important research
©Encyclopedia of Life Support Systems (EOLSS)
SOILS, PLANT GROWTH AND CROP PRODUCTION – Vol.III - Perennial Energy Crops: Growth and Management - M. Weih
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interest. He is participating in several EC projects. Author of more than 55 peer-reviewed publications in
scientific journals and books.
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